1. Field of the Invention
The present invention relates, in general, to dispersions, and, more particularly, to nano-engineered inks, methods to produce such inks, and their applications.
2. Relevant Background
Dispersions and inks are used in numerous applications. They are the building blocks of catalytic, electronic, electrical, magnetic, structural, optical, biomedical, chemical, thermal and consumer goods.
Inks are complex formulations that increasingly demand a balancing of often conflicting characteristics. Some of these characteristics include:
Particle size: Increasingly, inks that use sub-micron to nanoscale particles are desired. This is necessitated by ink jet heads and dots per inch (dpi) resolution requirements of the application. However, with increasingly smaller particles, a control of viscosity and prevention of flocculation becomes a challenging issue.
Particle density: For ceramic pigment-based lasting colors and in the case of metal inks, the particle density may be much higher than the solvent or medium they are dispersed in. In these cases, preventing particle settling down because of gravity and ensuring stability of the ink is often difficult.
Rheology: The rheology of dispersions is often very important to an application. Dispersions can be Newtonian or non-Newtonian. Rheology is often characterized through the viscosity, yield value, shear thinning and shear thickening properties of the dispersion. While pastes typically have higher viscosities (500 cP to 100000 cP), liquid inks typically have lower viscosities (the typical range is 5-150 cP). This means that particles used in the ink should have a size, shape and interconnectivity that enables appropriate viscosity.
Drying and Cure Rate: Drying and curing of a dispersion is often very important for applications. Some industrial applications require operational speeds in hundreds of feet per minute. Powders and ink formulations that impede these speeds or yield poor image are undesirable. Even in electronic and other device applications, faster cure of electrode ink or interconnect ink means better equipment utilization and superior productivity from labor and capital. There is a need for particle technology that serves this function. Fast cure times are often difficult to meet, because current low viscosity requirement necessitates the use of materials that cure slowly.
Pigment Wetting: Another important characteristic of inks is pigment wetting. This feature may be particularly important in durability applications. Therefore, powder engineering methods are desired that may be used to achieve reasonable pigment wetting combined with low viscosity.
Exterior Durability: Some applications desire very long durability, while others desire reasonable durability (in months). In particular, many pigments degrade in light or air or both with time. Powders used to formulate pastes or inks must meet appropriate exterior durability requirements.
Flexibility: The paste and ink formulations need to allow flexibility in the substrate they can be usefully applied to.
Adhesion: The formulation needs to adhere appropriately to the surface they are applied. In some cases, ease of cleaning and reuse may be desirable, while in others they need to last a long time.
VOC: In many applications, pastes and inks can cause environmental issues such as volatile organic compound (VOC) emissions when they are being used or cured or sintered. There is a need for formulations that reduce or eliminate VOC emissions.
Submicron powders and nanoscale powders offer opportunities to prepare novel dispersion (paste, ink) formulations. Nanopowders in particular, and sub-micron powders in general, are a novel family of materials whose distinguishing features include that their domain size is so small that size confinement effects become a significant determinant of the materials' performance. Such confinement effects can, therefore, lead to a wide range of commercially important properties. Nanopowders, therefore, are an extraordinary opportunity for design, development and commercialization of a wide range of devices and products for various applications. Furthermore, since they represent a whole new family of material precursors where conventional coarse-grain physiochemical mechanisms are not applicable, these materials offer unique combination of properties that can enable novel and multifunctional components of unmatched performance. Yadav, et al. in U.S. Pat. Nos. 6,228,904 and 6,344,271, which along with the references contained therein are incorporated herein by reference in their entirety, teach some applications of sub-micron and nanoscale powders.